Steady state tryptophan fluorescence emission intensity of G proteins increases 1.5-2.5 fold upon activation by GTP. We have used the two tryptophan G protein, transducin to analyze the origin of this effect. Absolute quantum yields for W127 and W207, were, determined by combining precise measurement of integral emission intensity calibrated against NATA (?=0.14) with the products of lifetime amplitude and exponential decay time for 295nm excitation and 345 nm emission. Four different ligand states of the protein with GDP, GTP?S, AlF4 and Mg2+ were examined in pH 7.0, 10 mM PO4 buffer at 220C. Two lifetimes dominated each state; one nearly constant at 3.210.1 ns and the other varying from 6.4ns for G[unreadable]GDP to 7.6 for G[unreadable]GTP?S[unreadable]Mg. ??of the 3.2ns state proved to be constant at 0.27 in all states while ??of the longer lived component ranged from 0.13 (G[unreadable]GDP) to 1.010.05 (G[unreadable]GTP?S[unreadable]Mg). We have identified W127 as the ?=0.27 ?=3.2 ns component. Failure of net steady state emis sion maximum to change despite the 8 fold increase in ?W207 with its apparent folding into a more apolar environment (crystal structure) implies a fast, non-radiative, powerful excited state energy conduit out of W207 in G[unreadable]GDP.